Selectively reinforced thermoformed article and process

ABSTRACT

A process is disclosed for producing reinforced twin-sheet thermoformed articles comprising the steps of heating first and second sheets of thermoplastic material; forming the sheets in molds; heating a reinforcing insert of thermoplastic material; placing the heated insert on the first sheet in its mold at the location to be reinforced; pressing the second sheet into contact with the heated first sheet and the heated insert to form the insert between the sheets so that a fusion of the thermoplastic sheets and the insert takes place to produce a reinforced section of the article having a desired shape and thickness. Alternative processes are disclosed wherein a heated quantity of thermoplastic liquid material is extruded or injected between the first and second sheets within the first and second molds either by inserting the liquid material before the molds are closed or by injecting it through one of the molds. Also disclosed are twin-sheet thermoformed articles with reinforced sections which are thicker than the combined thicknesses of the two thermoformed sheets at a particular location. The articles may advantageously be reinforced with plastic identical to the plastic of the two sheets. The articles are also easily recycled as they are entirely plastic with no non-plastic matter included.

This is a continuation of application Ser. No. 08/272,128, filed Jul. 8,1994, now abandoned, which was a continuation of application Ser. No.07/518,011 filed May 2, 1990 abandoned.

FIELD OF THE INVENTION

The present invention relates to twin-sheet thermoformed plasticarticles and processes for making such articles in general, and toselectively reinforced articles and twin-sheet thermoforming processesfor making such reinforced articles in particular.

BACKGROUND OF THE INVENTION

The thermoforming process may be used to form plastic articles of allsizes, and is particularly effective when it is desired to form largeplastic articles at low cost. Articles thermoformed from a single sheetof thermoplastic material are generally limited in wall thickness to thethickness of the sheet or less, throughout. By utilizing a twin-sheetthermoforming process articles of greater thickness and stiffness may beformed. The overall thickness of an article formed from twin-sheets maybe varied by providing a separation between the thermoformed sheets.However, solid portions of the article are limited to the combinedthicknesses of the two fused sheets from which the article is formed.Also, portions of the article which are formed more deeply in the moldwill tend to be thinner than the full combined thicknesses of the sheetsfrom which the article is formed, because the sheets must be drawn andstretched, with corresponding thinning, to form such portions.

Thus, although it is possible to form large and very stiff articlesusing the twin-sheet thermoforming process, the strength of the articlesis limited to the strength obtained by fusing the two sheets in variousconfigurations. Generally, if it is desired or necessary to increase thestrength of a portion of a formed twin-sheet article, it is necessary toincrease the thickness of one or both of the entire sheets, whichresults in higher overall cost of material. A need exists for atwin-sheet thermoformed article having reinforced portions which wouldstand up to greater wear or abrasion conditions, or facilitate efficientconnection of such articles.

It has been common practice to reinforce plastic twin-sheet thermoformedarticles with metal, wood, or other reinforcing material. Thesereinforcements may be costly and detract from the advantageousproperties of an all-plastic article, such as low density, fireresistance, and weatherability. Also, the addition of non-plasticreinforcements makes eventual recycling of the article difficult as thenon-plastic portions must be separated from the plastic before theplastic may be used again.

Methods for producing uniformly reinforced plastic sheet materialwherein reinforcing strands or webs are positioned between bonded sheetsare well known. A uniformly reinforced plastic sheet, however, resultsin an unnecessary increase in the overall weight and material usage ofthe finished article. A process has also been developed for selectivelyreinforcing portions of a single-sheet thermoformed article involvingthe application of powdered plastic to the sheet of thermoplasticmaterial prior to drawing the sheet in the vacuum mold. This processrequires complicated forms for retaining the reinforcing plastic on theplastic sheet during the heating step, and does not appear well adaptedfor twin-sheet thermoforming. Reinforcements employing plastic foams arealso known, but articles formed of different types of plastic are alsodifficult to recycle.

An improved thermoformed article and thermoforming process are neededwhich retain advantages and efficiencies of twin-sheet thermoforming yetallow localized increases in thickness of the finished article.

BRIEF DESCRIPTION OF THE INVENTION

The first step in the process for producing a reinforced article of thisinvention is to heat first and second sheets of thermoplastic material.The heated sheets are then formed in first and second molds. Areinforcing insert of thermoplastic material is heated and placed on thefirst sheet in the mold at the location to be reinforced. The heatedsecond sheet in the second mold is pressed into contact with the heatedfirst sheet and the heated insert within the first mold so that a fusionof the thermoplastic sheets and the insert takes place to produce anarticle having a reinforced section of greater thickness than thecombined thicknesses of the first and second sheets at the sectionlocation.

The process may alternatively include the steps of extruding a quantityof heated liquid thermoplastic material onto the first sheet in the moldin the area or areas of the article to be reinforced, or injecting aquantity of heated thermoplastic material onto the first sheet through afitting in the first mold or forming a hollow cavity between the firstand second sheets and injecting a liquid thermoplastic material throughone of the molds into the cavity.

It is an object of this invention to provide twin-sheet thermoformedplastic articles having reinforced areas of greater thickness than thecombined thicknesses at the location of the reinforcement of the sheetsfrom which the article is formed.

It is a further object of this invention to provide a process forproducing twin-sheet thermoformed articles with portions ofsignificantly greater thickness than the combined thicknesses, at aparticular location, of the sheets from which the article is formed.

It is a still further object of this invention to provide a process forthermoforming articles with reinforced sections of varying thickness.

It is also an object of this invention to provide a process for formingthermoformed articles with reinforced depressions and protrusions.

It is another object of this invention to provide a process for formingreinforced twin sheet thermoformed articles from a common thermoplasticmaterial to facilitate recycling.

Further objects, features, and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view in cross section of first and second heatedthermoplastic sheets formed in first and second molds with thereinforcing thermoplastic insert of the invention placed on the firstsheet within the first mold.

FIG. 2 is a schematic view in cross section showing the second sheet inthe second mold of FIG. 1 pressed into contact with the first sheet andthe heated insert in the first mold to form the reinforced twin-sheetthermoformed thermoplastic article of the invention.

FIG. 3 is a cross-sectional view of the integral reinforced twin-sheetthermoformed thermoplastic article formed in FIG. 2.

FIG. 4 is a schematic view in cross section of a quantity ofthermoplastic material being extruded onto a first sheet ofthermoplastic material in a mold according to an alternative embodimentof the process of the invention.

FIG. 5 is a schematic view in cross section of a quantity ofthermoplastic material being injected onto a first sheet within a firstmold through the wall of the first mold according to an alternativeembodiment of the process of the invention.

FIG. 6 is a schematic view in cross section of two thermoplastic sheetspressed into contact with one another by a pair of molds to form a shelldefining a cavity between the two sheets as a preliminary step in analternative embodiment of the process of the invention.

FIG. 7 is a schematic view in cross section of molten thermoplasticmaterial being injected into the cavity formed between the two sheets ofFIG. 6 through one of the molds according to the process of theinvention.

FIG. 8 is a schematic cross-sectional view of the reinforced twin-sheetthermoformed thermoplastic article formed within the molds of FIG. 6.

FIG. 9 is a cross-sectional view of the integral reinforced twin-sheetthermoformed thermoplastic article formed in FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1-9 wherein like numbers refer to similar parts,FIG. 1 shows a first sheet of thermoplastic material 30. The sheet 30will typically be an extruded thermoplastic sheet formed ofpolypropylene, polystyrene, polyethylene, acrylic, PETG, XT or ABS, orany other appropriate thermoplastic or alloy blend of thermoplastics.The sheet will typically be from 0.0075 to 0.500 inches thick and can beas large as the thermoforming equipment allows and the thermoformedarticle requires, usually up to seven by twelve feet. The first sheet 30is held along its edges and heated until it is in a malleable state. Themalleable sheet 30 is then positioned over a first vacuum mold 32 whichis a metallic one-piece mold with numerous passages (not shown) for airto be drawn from the interior of the mold.

When the first sheet 30 is placed over the first mold 32 the malleableplastic is drawn by vacuum down into the mold to conform to the contoursof the inner surface of the mold as in the standard one-sheetthermoforming process. The first sheet 30 may be formed, as shown inFIG. 1, to have an upwardly opening cavity.

As in the standard twin-sheet thermoforming process a secondthermoplastic sheet 34 is heated until malleable and drawn onto a secondvacuum mold 36. The second sheet 34 may be formed, as shown in FIG. 1,to have downwardly extending portions which open upwardly. Thedownwardly extending portions thus overlie the upwardly opening cavityof the first sheet. In the standard process the second sheet in thesecond mold would be pressed down over the first sheet in the first moldto cause the first sheet to fuse with the second sheet at specifiedpoints or surfaces.

In the process of this invention, however, a thermoplastic insert 38 isheated to a forming temperature in a preheat oven and positioned in thearea to be reinforced on the first sheet 30 in the first vacuum mold 32before the second sheet is fused with the first sheet as shown inFIG. 1. The insert 38 at forming temperature will be semi-solid andeasily placed by a robotic arm or by manual means into position in thefirst sheet 30 within the first mold 32. The insert 38, prior toheating, is shaped and sized to fit within the molded article which isto be reinforced. The insert may be machined, molded, or otherwiseprocessed to the appropriate dimensions. The insert 38 is heated in thepreheat oven at close proximity to the location of the first and secondmolds 30, 36 so that it may be rapidly extracted from the preheat ovenand placed in its correct position before the insert has time to cooland lose its malleability and formability. Once the sheets 30, 34, aredrawn into the molds 32, 36, they will begin to cool and loose theirmalleability. The actual time for the sheets to lose their malleabilitywill depend on the thermoplastic material, the temperature to which theyhave been heated, and the ambient temperature, but may be on the orderof 20 seconds. Thus, before the sheets 30, 34, and the insert 38 coolsignificantly, the second sheet 34 is pressed down onto the first sheet30 to place the insert 38 under continuous pressure as shown in FIG. 2.When the heated thermoplastic sheets are pressed into contact with oneanother a fusion of the thermoplastic material takes place producing anarticle 40 of continuous integral thermoplastic material. The insert 38,while still thermoplastic, is pressed between the first and secondsheets 30, 34 and is fused to both sheets. The fused plastic ispreferably cooled in the molds by liquid cooling of the molds. When thearticle 40 (shown in FIG. 3) is released from the molds 32, 36 thereinforced portion is an integral part of the plastic article.

The malleable insert 38, when subjected to pressure by the first andsecond sheets 30, 34 in the first and second molds 30, 36 is not onlyforced to fuse to the first and second sheets 30, 34 but is also formedto a desired shape, generally determined by the surfaces of the firstand second sheets. In general, the insert 38 will fill the space betweenthe two sheets to form an area of solid reinforced plastic.

Reinforcements may be desired at portions of the article which are drawnmore deeply in the mold to overcome the general thinning of the plasticsheets that occurs as the plastic is distorted to cover a greater depthin the mold. Such reinforcement may particularly be desired at high wearpoints of the article, such as in pallet feet, as disclosed in thecopending patent application directed to PLASTIC REINFORCED THERMOFORMEDPALLET, now U.S. Pat. No. 5,046,434. Reinforcement may also be desiredat portions of the article which must bear concentrated loads, such ashinge members, as disclosed in the copending patent application directedto a THERMOPLASTIC AQUATIC BIOMASS CONTAINMENT BARRIER WITH REINFORCEDHINGE, now U.S. Pat. No. 5,050,341.

As in the conventional twin-sheet thermoforming process, the two sheets30, 34 will generally be fused directly to one another at somenon-reinforced locations.

Although any appropriate thermoplastic materials may be used for theinserts and the sheets, if eventual recycling of the plastic article isenvisioned, the three elements of the article are advantageously formedof the same thermoplastic material. An article so formed may, at the endof its useful life, be conveniently and expeditiously ground up intouniform thermoplastic chips and recycled.

An alternative embodiment of the process of this invention isillustrated in FIG. 4. A first sheet 30 of thermoplastic material isheated and drawn into a vacuum mold. Once the first sheet has beenmolded and while it is still hot an injector 37 is positioned within thecavity to be reinforced and a quantity of heated thermoplastic material42 is extruded into the cavity. Heated supply lines (not shown) bringhot liquid thermoplastic to the injector 37 which profiles the hotliquid through a die of the desired cross section. Once the hotthermoplastic has been extruded onto the first sheet, a second sheetwhich has been drawn onto a second vacuum mold is pressed down on thefirst sheet 30 in the first vacuum mold 32 and made to fuse with thefirst sheet 30 and the extruded quantity of thermoplastic material 42.The pressure of the sheets on the extruded quantity of thermoplasticmaterial 42 will form the quantity to the desired shape and thicknessrequired for the reinforcement.

The extrusion process is advantageously employed for reinforcingportions of the thermoformed article which have a consistent profile.

Alternatively, as shown in FIG. 5, the quantity of liquid thermoplasticmaterial 42 may be injected onto the first sheet 30 by an injector 41introduced through a passage 43 in a first mold 39. The injector 41pierces the maleable first sheet 30 within the first mold 39 andintroduces a specified quantity of thermoplastic material 42. Theinjector is then withdrawn and a second sheet, which has been formed ina second mold, is pressed into contact with the first sheet 30 and thequantity of thermoplastic material 42 to form the quantity 42 and fusethe quantity and the sheets into a solid article.

In another embodiment of the process of this invention, illustrated inFIGS. 6-9, the first and second thermoplastic sheets 30, 34 are heateduntil malleable and then formed in first and second vacuum molds 44, 46at substantially the same time. The two sheets are then forced togetheras shown in FIG. 6 and fused at the areas of contact. The first mold 44molds a first portion of a shell 45 in the first sheet 30 which thusforms an upwardly opening cavity, and the second mold 46 molds a secondportion of a shell 47 in the second sheet 34 having portions which opendownwardly toward said cavity. When the first sheet 30 and the secondsheet 34 are pressed together a continuous shell 49 is formed defining ahollow cavity 50. To achieve the reinforcement a quantity of heatedliquid thermoplastic material 48 is injected under pressure into thecavity 50 between the first and second sheets by an injector 52 insertedthrough a passage 51 in either the first or second vacuum mold andthrough the sheet formed thereon. For illustrative purposes, the passage51 is shown in the second mold 46. The quantity of liquid thermoplasticmaterial 48 injected into the cavity 50 is sufficient to substantiallyfill the cavity 50 with thermoplastic material. The cavity 50 may bevented in a conventional manner to allow escape of air. Under appliedpressure the liquid thermoplastic material 48 then fuses with the firstsheet 30 and the second sheet 34 as shown in FIG. 8 to form an integralsolid plastic article 54 as shown in FIG. 9. This process is mostadvantageously employed if the area of reinforcement extends above thesurface of the first sheet, is of varying cross section, or otherirregular shape.

It should be noted that the reinforced thermoforming process of thisinvention may be performed using well known twin-sheet thermoformingequipment. Articles formed by this process may have one or severalreinforced portions. Although for purposes of recycling it is preferablethat the reinforcing thermoplastic be the same as the thermoplastic ofthe first and second sheets, in some applications it may be advantageousto utilize a reinforcing thermoplastic material with differentproperties than the surrounding sheets.

It is to be understood that the invention is not confined to theparticular construction and arrangement of parts herein illustrated anddescribed but embraces such modified forms thereof as come within thescope of the following claims.

We claim:
 1. A reinforced twin-sheet thermoformed thermoplastic articlecomprising:(a) a first thermoformed thermoplastic sheet; (b) a secondthermoplastic sheet; and (c) a quantity of solid thermoplastic materiallocated between and fused to the first and second thermoplastic sheetsat a reinforced section of the article such that the reinforced sectionis of greater thickness than the combined thickness of the first andsecond sheets at the section location, and wherein the firstthermoplastic sheet is fused to the second thermoplastic sheet withoutan intermediate sheet at locations other than at the reinforced sectionof the article.
 2. The article of claim 1 wherein the first sheet, thesecond sheet, and the quantity of thermoplastic material are all formedof the same thermoplastic material.
 3. A reinforced twin-sheetthermoformed thermoplastic article comprising:(a) a first thermoplasticsheet having at least a portion thereof thermoformed; (b) a secondthermoplastic sheet having at least a portion thereof thermoformed,wherein the thermoformed portions of the first and second sheets definea cavity; and (c) a solid thermoplastic reinforcing insert molded tosubstantially the same shape as the cavity and fused to the thermoformedportions of the first and second sheets.
 4. The article of claim 3wherein the first sheet, the second sheet and the insert are all made ofthe same thermoplastic material.
 5. The article of claim 3 wherein thefirst and second thermoplastic sheets are fused directly to one anotherin at least one location.
 6. A reinforced twin-sheet thermoformedthermoplastic article comprising:(a) a first thermoformed thermoplasticsheet formed to have an upwardly opening cavity; (b) a secondthermoformed thermoplastic sheet having downwardly extending portionswhich open upwardly, said downwardly extending portions overlying theupwardly opening cavity of the first sheet, wherein portions of thefirst sheet are fused directly to portions of the second sheet spacedfrom the cavity; and (c) a quantity of solid thermoplastic materialdisposed within the first thermoplastic sheet cavity and fused to boththe first thermoplastic sheet and the second thermoplastic sheet andmolded into the shape of the cavity by the first sheet and the secondsheet.
 7. A reinforced twin-sheet thermoformed thermoplastic articlecomprising:(a) a first thermoformed thermoplastic sheet formed to havean upwardly opening cavity; (b) a second thermoformed thermoplasticsheet having portions which open downwardly toward said cavity, saiddownwardly opening portions overlying the upwardly opening cavity of thefirst sheet, wherein portions of the first sheet are fused directly toportions of the second sheet spaced from the cavity; and (c) a quantityof solid thermoplastic material disposed within the cavity and fused toboth the first thermoplastic sheet and the second thermoplastic sheetand molded into the shape of the cavity by the first sheet and thesecond sheet.